1. Technical Field
The present invention relates to a plasma display panel used in a display device or the like.
2. Background Art
The definition and screen size of plasma display panels (hereinafter referred to as “PDPs”) can be increased, so that televisions or the like of 65 inch size are manufactured. Recently, PDPs have been applied to high definition televisions where the number of scan lines is two or more times that of a conventional National Television Standards Committee (NTSC) system, and PDPs containing no lead component are demanded in consideration of environmental issues.
A PDP is essentially formed of a front plate and a rear plate. The front plate has a glass substrate, a display electrode, a dielectric layer, and a protective layer. The glass substrate is sodium borosilicate glass produced by a float method. The display electrode includes a stripe-like transparent electrode and bus electrode formed on one surface, namely the main surface, of the glass substrate. The dielectric layer covers the display electrode and works as a capacitor. The protective layer is made of magnesium oxide (MgO) formed on the dielectric layer. While, the rear plate has the following elements:                a glass substrate;        a stripe-like address electrode formed on one surface, namely the main surface, of the glass substrate;        a base dielectric layer for covering the address electrode;        barrier ribs formed on the base dielectric layer; and        phosphor layers that are formed between barrier ribs and emit red, green, and blue lights, respectively.        
The front plate and the rear plate are hermetic sealed while the electrode forming sides of them are faced to each other. Discharge gas of Ne—Xe is filled into discharge space partitioned by the barrier ribs at a pressure of 400 to 600 Torr. The PDP selectively applies video signal voltage to the display electrode to perform discharge, and the ultraviolet rays generated by the discharge excite respective phosphor layers to emit red, green, and blue lights, and thus achieve color image display (patent document 1).
An attempt to improve the electron emission characteristic by mixing an impurity into the protective layer in the PDP is performed. When the impurity is mixed into the protective layer to improve the electron emission characteristic, however, charge is simultaneously accumulated on the surface of the protective layer, and the attenuation rate, at which the charge decreases with time, increases when the protective layer is used as a memory function. Therefore, in order to suppress the increase, measures of increasing the applied voltage or the like are required. The protective layer is required to have two following contradictory characteristics: high electron emission performance and high charge retention characteristic. The charge retention characteristic means reduction in attenuation rate of charge when the protective layer is used as a memory function.